A fundamental engineering challenge in designing a holographic video display is achieving a high enough space-bandwidth product to meet the image size and view angle requirements of the viewer. In general, a large view angle is possible only with very small diffraction fringes (and thus small pixels), while a large image requires a large light modulator. In simple terms, what is therefore necessary is a massive number of very small pixels. In some cases it is possible to use optics to trade off one of these for the other, such as, for example, by magnifying a display that is higher-resolution than needed or by demagnifying a large modulator to get a small enough effective pixel size, but passive optics cannot simultaneously increase both size and angle.
Because of the practical limitations on devices that can currently be fabricated, it is commonly necessary to use either, or both, scanning (re-using a smaller device for more than one region of the image) or tiling (using multiple copies of a small device) [see, for example, K. Sato, A. Sugita, M. Morimoto, and K. Fujii, “Reconstruction of Color Images at High Quality by a Holographic Display,” Proc. SPIE Practical Holography XX, 6136, 2006; C. Slinger, C. Cameron, S. Coomber, R. Miller, D. Payne, A. Smith, M. Smith, M. Stanley, and P. Watson, “Recent Developments in Computer-Generated Holography: Toward a Practical Electroholography System for Interactive 3D Visualization,” Proc. SPIE Practical Holography XVIII, 5290, pp. 27-41, 2004]. For example, Son, Shestak, et al. try to solve the problem of making the scan line stationary while the pattern is traveling across the light modulator by using a pulsed laser, in order to obtain a “snapshot” of the moving pattern. The resulting scan line is too short, so six snapshots are pasted together end-to-end using six mirrors in a stepped arrangement [J-Y. Son, Jung-Young, S. A. Shestak, S-K. Lee, and H-W. Jeon, “Pulsed laser holographic video”, Proc. SPIE, vol. 2652, pp. 24-28].
A prior 2-D diffractive display architecture, dating from the 1930s, is called the Scophony system. In a 2-D Scophony display, an electrical sinusoidal oscillation is converted to a compression wave that changes the index of refraction in some material and thus creates a sinusoidal phase grating. Amplitude-modulating this sinusoidal carrier with a video signal changes the amplitude of a diffracted beam of light, which is then scanned by rotating or oscillating mirrors to form a video image [H. W. Lee, “The Scophony Television Receiver,” Nature, 142, 3584, pp. 59-62, 1938]. Besides the need for a monochromatic light source to enable sharp focus, the major limitation of a Scophony display system stems from the fact that the grating pattern is moving with the speed of sound through the diffractive material. To create a stable image, the diffracted light must therefore be imaged in a mirror moving in the opposite direction, a requirement that makes scaling such a system difficult.
Two earlier generations of displays developed by the MIT Media Laboratory were variations on the Scophony system. If a Scophony-type display is not driven with a single amplitude-modulated sinusoid, but rather with a superposition of many gratings at different frequencies, it can output light in multiple directions. The output of an acousto-optic modulator (AOM) can then be treated as one “holo-line” of a horizontal-parallax-only (HPO) holographic image. The first-generation MIT display, known as the “Mark I” [P. St.-Hilaire, S. A. Benton, M. Lucente, M. L. Jepsen, J. Kollin, and H. Yoshikawa, “Electronic Display System for Computational Holography,” Proc. SPIE Practical Holography IV, 1212, pp. 174-182, 1990], is depicted in FIG. 1. As shown in FIG. 1, the Mark I is fundamentally a standard Scophony architecture, with light from laser light source 105 being diffracted by a 50 MHz bandwidth TeO2 AOM 110 driven by a 32,768×192 raster. The video signal employed is multiplied by a 100 MHz sinusoid and lowpass filtered to retain the lower sideband. The view volume is 25 mm×25 mm×25 mm (W×H×D) and the view angle is 15°. The signal passes through transform lens 120, and then is scanned by vertical scanner 130 and horizontal scanner 140. Vertical scanner 130 is a galvanometer and horizontal scanner 140 is a polygonal mirror. Holographic image 150 is rendered through output lens 160. A Thinking Machines CM2 (not shown) performs the computation.
In order to scale up the image size so that both of a viewer's eyes could fit into the view zone with some added look-around, St.-Hilaire et al. increased the space-bandwidth product of the system by using 18 TeO2 AOM channels in parallel, thus outputting a group of 18 adjacent scan lines, resulting in the “Mark II” architecture [P. St.-Hilaire, S. A Benton, M. Lucente, J. D. Sutter and W. J. Plesniak, “Advances in Holographic Video,” Proc. SPIE Practical Holography VII, 1914, pp. 188-196, 1993] shown in FIG. 2. In FIG. 2, the light diffracted from laser light source 205 by AOMs 210 passes through transform lens 215 and toroidal lens pair 220 before being scanned by vertical scanner 230. Vertical scanner 230 moves in 18-line steps to scan out 144 lines, each having 262,144 samples. The view volume is 150 mm×75 mm×150 mm and the view angle is 30°. The signals then pass through vertical relay lenses 240, 245 to beamsplitter 250. Because of the difficulty of making a single horizontal scanner wide enough to meet the requirements, Mark II uses a synchronized linear array 260 of galvanometric scanners 265. Holographic image 270 is rendered through output lens 280. The 18 video channels were initially generated by a compact dataflow computer called Cheops [J. A. Watlington, M. Lucente, C. J. Sparrell, V. M. Bove, Jr., and I. Tamitani, “A Hardware Architecture for Rapid Generation of Electro-Holographic Fringe Patterns,” Proc. SPIE Practical Holography IX, 2406, pp. 172-183, 1995], and in later work the display was driven by three dual-output PC video cards [V. M. Bove, Jr., W. J. Plesniak, T. Quentmeyer, and J. Barabas, “Real-Time Holographic Video Images with Commodity PC Hardware,” Proc. SPIE Stereoscopic Displays and Applications, 5664A, 2005]. The use of parallel AOMs and a segmented horizontal scanner gives the Mark II a modular character that allows scale-up of the system, albeit at the expense of more video input channels and more synchronized mirror-drive circuitry.
One goal of research in holographic video has been constructing a display suitable for use by consumers. Unlike earlier systems, such a display must be at least standard television resolution, quiet, reliable, compact, manufacturable for at most a few hundred dollars, and capable of being driven by the graphics hardware of a PC or game console, rather than requiring specialized hardware. A vast amount of 3-D visual data now exists, particularly in the gaming world (though most is rendered for 2-D viewing), and three-dimensional displays could easily take advantage of this resource if they could be manufactured inexpensively. The widespread adoption of such displays would also spark innovation in 3-D capture of real-world scenes.